Remove tokio/net feature from client feature

[erickt]

Before this patch, hyper-util's feature client depended on tokio's feature net, which in turn depends on mio. That causes hyper-util to fail to compile on platforms like Fuchsia, which don't support mio. So this patch instead moves the tokio/net dependency into the tokio feature to avoid this issue.

Unfortunately though I think this is a breaking change, since now users might need to add the tokio feature. So either we could:

1. Bump the major version.
2. We could add some new features that we could use that parallel client and client-legacy. For example, client-without-tokio and client-legacy-without-tokio, and replace all the instances of feature = "clienty" feature checks with any(feature = "client", feature = "client-legacy-without-tokio").

What would you like us to do?

[seanmonstar]

Hm, I think tokio/net was required for the legacy client, specifically the HttpConnector. Everything else probably didn't quite need it, right? Or are you hoping to enable the whole legacy client without mio?

[erickt]

Yeah, I'm trying to get us back to the point where we can use hyper-rustls on Fuchsia, and it's taking a bit to untangle our dependencies. The crux of our problem is we don't support mio on Fuchsia, and instead use our own Fuchsia-specific executor. We're currently stuck on hyper 0.14.x, and rustls-hyper 0.24.x, since those versions allow us to disable the tokio runtime, which depends on mio.

As far as I can tell, hyper-rustls doesn't really need to be aware of the tokio runtime, but happens to depend on it because of hyper-util's HttpConnector isn't generic enough. Maybe we could restructure that library to allow the dns resolver and socket creator to be generic, and have a wrapper that passes in the tokio specific items?

[seanmonstar]

Hm, the HttpConnector uses tokio::net::TcpStream to create connections, so it needs tokio/net. It hasn't been able to work without it, even in 0.14.x.

[erickt]

It looks like hyper-rustls 0.24 only required hyper 0.14's client feature, and had an optional tokio-runtime feature that depended on hyper-runtime, which depended on tokio/net. That changed when they updated to hyper 1.x in rustls/hyper-rustls#232.

Looking at hyper-rustls use of hyper-util, it looks like they really are only using:

1. hyper_util::rt::TokioIo - They use this trait to adapt the network traffic to tokio-rustls. As far as I can tell, tokio-rustls is only using tokio's IO traits so it wouldn't be an issue for Fuchsia.
2. hyper_util::client::legacy::Connection - by itself this trait is pretty trivial, but it needs the legacy-client feature, which needs client feature, which depends on tokio/net.
3. hyper_util::client::legacy::HttpConnectorBuilder - this uses HttpConnector, which ends up pulling in the tokio network types, and thus mio.

So this patch was an attempt to fix (2). For (3), maybe we could put all HttpConnectorBuilder functionality behind a feature flag in hyper-rustls?

This all is a bit ugly though. Do you have any suggestions on what might work better?